1. a) Field of the invention
The present invention relates to a compact zoom lens system which is to be incorporated in lens shutter cameras and so on.
2. b) Description of the prior art
Under the recent trend to use more and more compact cameras, it is necessary to develop zoom lens systems which are more compact in dimensions and lighter in weight. Especially, a strong demand is posed for a compact zoom lens system which is to be used in the so-called lens shutter cameras allowing no exchange of lens systems, and has a vari-focal ratio of 2 to 3 and a simple composition.
It is unnecessary for such a zoom lens system to be incorporated in the lens shutter cameras, unlike the lens systems to be used in the single lens reflex cameras, to reserve a specific back focal length. When the zoom lens system to be incorporated in the lens shutter cameras has too short a back focal length, however, the lens component arranged on the image side in the zoom lens system will have a large diameter, thereby making it impossible to design the lens system compact. Further, flare is apt to be produced by detrimental rays in this zoom lens system.
As a zoom lens system which is designed taking this point into consideration and capable of having favorable imaging performance, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-153511. This zoom lens system is composed, in the order from the object side, of a first lens unit having a positive refractive power, a second lens unit having a positive refractive power and a third negative lens unit, adapted so as to perform variation of focal length by varying the airspaces reserved between the lens units along the optical axis, and designed so as to satisfy the following conditions: EQU 0.2&lt;.PHI..sub.I /.PHI..sub.W &gt;0.9 EQU 1.1&gt;.PHI..sub.12W /.PHI..sub.W &gt;1.9 EQU 1.5&gt;.beta..sub.3T /.beta..sub.3W&gt; 2.8
wherein the reference symbol .PHI..sub.w represents the refractive power of the zoom lens system as a whole at the wide position, the reference symbol .PHI..sub.1 designates the refractive power of the first lens unit, the reference symbol .PHI..sub.12W denotes the total refractive power of the first lens unit and the second lens unit at the wide position, the reference symbol .sym..sub.3W represents the lateral magnification of the third lens unit at the wide position, and the reference symbol .beta..sub.3T designates the lateral magnification of the third lens unit at the tele position. PA1 wherein the reference symbol D1 represents the thickness of the first lens unit as measured on the optical axis and the reference symbol IH designates the length equal to half the diagonal line of the image surface. PA1 wherein the reference symbols R.sub.22 and R.sub.23 represent the radii of curvature on the second surface and the third surface as counted from the object side in the second lens unit, and the reference symbol .PHI..sub.W designates the refractive power of the zoom lens system as a whole at the wide position. PA1 wherein the reference symbol .PHI..sub.2R represents the refractive power of the image side surface of the air lens and the reference symbol .PHI..sub.2 designates the refractive power of the second lens unit. PA1 wherein the reference symbol H.sub.2 represents the distance between the principal points of the second lens unit. PA1 wherein the reference symbols v.sub.p and v.sub.n represent the Abbe's numbers of the positive lens element and the negative lens element respectively arranged in the first lens unit, the reference symbol r.sub.1 designates the radius of curvature on the most object side surface arranged in the first lens unit, and the reference symbol .PHI..sub.w denotes the refractive power of the zoom lens system as a whole. PA1 wherein the reference symbol .PHI..sub.w represents the refractive power of the zoom lens system as a whole at the wide position, the reference symbol .PHI..sub.l designates the refractive power of the first lens unit, the reference symbol .PHI..sub.12w denotes the total refractive power of the first lens unit and the second lens unit at the wide position, and the reference symbols .beta..sub.3w and .beta..sub.3T represent the lateral magnifications of the third lens unit at the wide position and the tele position respectively.
Furthermore, the zoom lens system proposed by Japanese Patent Kokai Publication No. Hei 1-230013 has a composition similar to that of the zoom lens system disclosed by the above-mentioned Japanese Patent Kokai Publication No. Sho 63-153511, is adapted so as to perform variation of focal length by moving the first lens unit and the third lens unit integrally, and adopts a lens barrel having a simplified structure.
Moreover, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-16515 consists, like the conventional examples described above, of a first positive lens unit, a second positive lens unit and a third negative lens unit which are arranged in order from the object side. In this zoom lens system, an aperture stop is arranged outside the second positive lens unit for simplifying structure of the lens barrel.
However, each of the conventional zoom lens systems described above has a complicated composition and is not sufficiently compact.